The primary function of phytoplankton in aquatic ecosystems is to perform photosynthesis, converting sunlight into energy and producing oxygen as a byproduct. Phytoplankton, which are microscopic organisms, form the base of the aquatic food web, serving as a primary food source for small marine animals. They play a crucial role in carbon fixation, absorbing carbon dioxide from the water and contributing to the regulation of global climate through the carbon cycle. The health and abundance of phytoplankton populations are essential for maintaining the productivity and balance of aquatic ecosystems.